1. Field of the Invention
The present invention relates to a metal material having a photocatalytic activity and a method of manufacturing the metal material, and more particularly, to a metal material containing titanium having a high photocatalytic activity on the surface thereof and having excellent designing properties and a method of manufacturing the metal material.
2. Description of the Related Art
It has been known that an n-type semiconductor such as titanium oxide exhibits a photocatalytic function with light having a specific wavelength and that this n-type semiconductor has deodorizing and antifungal functions due to its strong oxidizing function.
Conventionally, as photocatalysts having a semiconductor function, there have been used titanium oxide, iron oxide, tungsten oxide, silicon oxide, etc. or these oxides bearing a metal such as platinum for the purpose of improving their catalytic functions. Further, in order to utilize the deodorizing and antifungal functions of these materials, these materials are divided into fine particles and a fixed layer is formed on the surface of the fine particles or the fine particles are dispersed into an object material to be processed.
As methods for coating a finely divided photocatalyst onto a substrate in order to provide a metal material with deodorizing and antifungal functions by utilizing the photocatalyticic function, a spray coating method, a dip coating method and a spin coating method are known. However, a layer formed by coating a fine-particle dispersion lacks uniformity from a microscopic observations. Therefore, the layer has such a problem that the efficiency of the photocatalytic function is poor, the layer strength is not sufficient and the adhesiveness to a substrate is not sufficient so taht the layer has insufficient stripping resistance resulting in the layer being damaged or peeling off with time.
In order to achieve a sufficient strength of a thin layer having the photocatalytic activity, there has also been proposed a method of anodically oxidizing a metal material substrate composed of titanium or a titanium alloy to form a titanium oxide layer on the surface of the substrate. It is known that an anodized layer formed on the metal material including titanium exhibits various colors depending on the layer thickness. Since the thickness of an anodized layer is approximately proportional to an anodic oxidation potential, it is possible to obtain metal materials having various colors by controlling the oxidation potential and to obtain a metal material having a desired color to meet objectives by adjusting the oxidation potential, and the metal materials thus obtained are desirable in light of designing.
It has been known that the photocatalytic activity is revealed by heating an anodized layer at a predetermined temperature and a photocatalytic characteristic can be controlled by the heat processing conditions. However, there has been a problem that when the heat processing is carried out at a temperature above 600.degree. C. which is an optimum temperature for achieving an excellent photocatalytic activity, the thickness of the anodized layer formed becomes thick and forms a grey color hue so that when the metal material is used as a construction material, the designing property is seriously deteriorated.